The scientists decided to use a two-stroke engine which doesn't have separate intake/exhaust strokes. Instead, the piston movement uncovers ports during the cycle which simultaneously draws a fresh charge of fuel/air through one port and forces the exhaust gas out another port. In terms of emissions, this engine is not as "clean" as the four-stroke engine, but it is simple enough to be constructed using standard MEMS processes. Another thing the developers did to simplify the engine manufacturing was using a flat, oval-shaped chamber and triangular rotor. As the rotor rotates within the chamber, the edges of the rotor act to partition areas of the chamber into smaller volumes where combustion can occur. Combustion acts to further rotate the rotor around the chamber, transferring force to the shaft that is attached to the rotor.